Variable displacement hydraulic pumps are commonly used to provide adjustable flows of pressurized fluid to machine actuators, for example to cylinders or motors associated with moving machine tools or linkages. Based on a demand of the actuators, the displacement of the pump is either increased or decreased such that the actuators move the tools and/or linkage at an expected speed and/or with an expected force.
Typical variable displacement pumps used in hydraulic tool systems are known as swashplate-type pumps. This type of pump includes a plurality of plungers held against a plunger engagement surface of a tilted swashplate. In most situations, the swashplate is generally planar and includes a smooth driving surface. A joint such as a ball and socket joint is disposed between each plunger and the engagement surface to allow for relative movement between the swashplate and the plungers. Each plunger is slidably disposed to reciprocate within an associated barrel as the plungers and tilted surface of the swashplate rotate relative to each other. As each plunger is retracted from the associated barrel, low-pressure fluid is drawn into that barrel. When the plunger is forced back into the barrel by the plunger engagement surface of the swashplate, the plunger pushes the fluid from the barrel at an elevated pressure. In this configuration, the output of the pump can be varied by adjusting a tilt angle of the swashplate.
Historically, the swashplate of a pump has been tilted to a desired angle by one or more actuators connected to a side of the swashplate. As the actuator is extended or retracted, the swashplate is caused to tilt about a pivot axis. One or more solenoid-operated valves associated with the swashplate are controlled in response to various inputs to either direct pressurized fluid to the actuator to extend the actuator, or to drain fluid from the actuator to retract the actuator, thereby adjusting the tilt angle of the swashplate.
Although functionally adequate to control the pump, the solenoid-actuators described above may be problematic in some situations. For example, when power to the solenoid-actuators fails or when input used to control the solenoid-actuators is faulty, the tilt angle of the swashplate may be improperly adjusted or not adjusted at all.
One attempt to improve pump displacement control is described in U.S. Pat. No. 4,194,361 (the '361 patent) issued to Pahl et al. on Mar. 25, 1980. Specifically, the '361 patent describes a swashplate pump having a group of solenoid valves that can control displacement of the pump during normal operation, and a manual valve that can override the group of solenoid valves and control displacement of the pump during emergency conditions. The manual valve is linked to a mechanical override that is movable by an operator. During normal operation, the mechanical override is maintained in a deactivated state by spring pressure, and a controller communicates with the group of valves to adjust displacement of the pump in response to a manual input and a displacement position of the pump. During emergency conditions, for example when electrical failure has occurred, the mechanical override can be moved by a human operator to control displacement adjustments of the pump via the manual valve.
Although the '361 patent may provide for pump control during emergency conditions, the provided control may be limited. That is, the '361 provides for only manual control during the emergency conditions, and there may be some situations when manual control is insufficient or undesired.
The disclosed hydraulic control system is directed to overcoming one or more of the disadvantages set forth above and/or other problems of the prior art.